This application claims priority under 35 U.S.C. xc2xa7xc2xa7119 and/or 365 to 9713807.7 filed in United Kingdom on Jun. 30, 1997; the entire content of which is hereby incorporated by reference.
This invention relates to a mobile communications system, and in particular to a base station, and a method of operation thereof, intended to provide information for use in network planning.
A cellular mobile telephone system consists of a number of base stations, a number of mobile stations and at least one mobile switching centre (MSC).
The geographical area in which all mobile stations are connected to the same base station is called a cell. The number of cells in a network is normally greater than the number of available frequencies. The same frequency must therefore be used by a number of cells. To prevent interference between cells using the same frequency, the assignment of frequencies to different cells must be carefully planned.
When a mobile station moves around in the network, it must sometimes change the base station to which it is connected. This procedure is called handover. It is not possible to perform handover between any cells in the network. For each cell, a limited number of other cells, to which handover is possible, must be defined in the system. These cells are called neighbours or neighbouring cells to the first cell, and must naturally operate on different frequencies from the first cell.
At every handover attempt there is always a risk that the signal quality is insufficient for the mobile station to establish a new radio connection with any of the defined neighbouring cells. This will result in dropped calls. If it is possible to increase the number of defined neighbours, this would of course increase the possibility to accomplish the handover successfully.
A mobile station must always take measurements on signal quality and signal strength for all the defined neighbouring cells, otherwise it would not be possible to decide to which of the neighbouring cells a possible handover should be performed. For each new neighbouring cell relation we define for a cell, we must increase the number of frequencies for every mobile station in that cell to take measurements on. This will increase the load on the mobile stations and thereby reduce the quality of the measurements.
Here we have a conflict, too many defined neighbouring cells gives less risk of dropped calls, but it will increase the system load. On the other hand, if we define too few neighbours, we will have a low system load, but the risk of losing calls will be too high. It is essential in network planning to always define the best neighbours, and not too many to each cell in the network.
The making of power and quality measurements is defined in, for example, the GSM system in GSM Technical Specification 04.08.
To summarise, there are two major problems in network planning. First, there is frequency planning. This means making decisions as to which frequency or frequencies to assign to each cell in the network. This must be done very carefully, in order to maximise the system capacity, at the same time as the interference level between cells using the same frequency must be kept to a minimum. The second problem is to define the best neighbouring relations, for all cells in the network. Both these problems are hard for the operator, or network administrator, to deal with, and every tool, or mechanism, that could make it easier for the operator to do this planning is surely appreciated.
The main purpose with the invention is to deal with the optimal neighbouring relation problem.
It is an object of the present invention to use a technique, similar to that described above, to provide additional information to a network operator about possible neighbouring relations which might be defined.
It is a further object of the invention to determine new channel allocations which might improve the performance of the network.
Specifically, in addition to directing mobile stations to make measurements on the channels used by the known neighbouring cells, a base station also directs mobile stations to make measurements on other channels in use within the network. Instructions are sent to the mobile stations from the base stations, but the intelligence which decides on the additional measurements to be made may reside in a base station controller, which is connected to several base stations. As used herein, the term xe2x80x9cbase stationxe2x80x9d is intended to encompass the device located in a specific cell, as well as any base station controller which originates any messages controlling that device.
The results of these measurements can be used to obtain statistics about signal quality within a cell from many or all other cells in the network. These statistics can be used to find new neighbouring relations which might usefully be defined.
The information obtained thereby can be used to improve the performance of the cellular network.